ci: fix/mitigate `/root/buildinfo`

[cgwalters]

Our CI now broke with

+ bootc container lint --fatal-warnings
error: Linting: Checks failed: 1
Lint warning: var-tmpfiles: Found content in /var missing systemd tmpfiles.d entries:
  d /var/roothome/buildinfo 0755 root root - -
  d /var/roothome/buildinfo/content_manifests 0755 root root - -
Found non-directory/non-symlink files in /var:
  var/roothome/buildinfo/content_manifests/content-sets.json

See https://gitlab.com/redhat/centos-stream/containers/bootc/-/issues/1195

[cgwalters]

OK and now we have another different failure

thread 'main' panicked at /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/gio-0.20.12/src/gio_future.rs:66:13:
Spawning futures only allowed if the thread is owning the MainContext
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Error: nu::shell::core_dumped

  x External command core dumped
     ,-[/var/ARTIFACTS/work-test-20-local-upgradeogm5yztq/tmt/plans/integration/test-20-local-upgrade/discover/default-0/tests/tmt/tests/booted/test-image-pushpull-upgrade.nu:142:14]
 141 |     # Verify we staged an update
 142 |     let st = bootc status --json | from json
     :              ^^|^^
     :                `-- core dumped with SIGABRT (6)
 143 |     let staged_digest = $st.status.staged.image.imageDigest
     `----

Connection to 13.59.156.65 closed.

[jeckersb]

(xref #1544 (comment))